异步处理的业务：邮件提醒、短信提醒等。
e.g. 用户登录时添加短信通知他的粉丝
The whole process depends on sendMsgNotify. Once sendMsgNotify fails or crushes, the login will fail. By the way, we can catch the exception by try-catch.
邮件是否发送依赖于其他模块是否执行成功，如果其他模块执行出现延迟或者失败，邮件则无法发送成功。
In fact, we can use Async (create another thread to send notification) to complete the function, which will not influence the login functionality.
